Investments - Composition of the Company's investment portfolio by geographic region, at amortized cost and fair value (with corresponding percentage of total portfolio investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Investments
Amortized Cost	$ 1,535,660	$ 1,465,352
Fair Value	$ 1,543,910	$ 1,468,996
Amortized Cost, Percentage of total portfolio investments	100.00%	100.00%
Fair Value, Percentage of total portfolio investments	100.00%	100.00%
International
Investments
Amortized Cost	$ 62,975	$ 62,244
Fair Value	$ 60,203	$ 59,706
Amortized Cost, Percentage of total portfolio investments	4.10%	4.30%
Fair Value, Percentage of total portfolio investments	3.90%	4.10%
Midwest
Investments
Amortized Cost	$ 330,223	$ 312,637
Fair Value	$ 331,176	$ 313,240
Amortized Cost, Percentage of total portfolio investments	21.50%	21.30%
Fair Value, Percentage of total portfolio investments	21.40%	21.30%
Northeast
Investments
Amortized Cost	$ 274,847	$ 245,445
Fair Value	$ 275,666	$ 244,305
Amortized Cost, Percentage of total portfolio investments	17.90%	16.70%
Fair Value, Percentage of total portfolio investments	17.90%	16.60%
Southeast
Investments
Amortized Cost	$ 374,990	$ 353,079
Fair Value	$ 383,322	$ 360,465
Amortized Cost, Percentage of total portfolio investments	24.40%	24.10%
Fair Value, Percentage of total portfolio investments	24.80%	24.50%
Southwest
Investments
Amortized Cost	$ 186,280	$ 183,722
Fair Value	$ 185,255	$ 181,981
Amortized Cost, Percentage of total portfolio investments	12.10%	12.50%
Fair Value, Percentage of total portfolio investments	12.00%	12.40%
West
Investments
Amortized Cost	$ 239,494	$ 241,386
Fair Value	$ 240,354	$ 242,073
Amortized Cost, Percentage of total portfolio investments	15.60%	16.50%
Fair Value, Percentage of total portfolio investments	15.60%	16.50%
Northwest
Investments
Amortized Cost	$ 66,851	$ 66,839
Fair Value	$ 67,934	$ 67,226
Amortized Cost, Percentage of total portfolio investments	4.40%	4.60%
Fair Value, Percentage of total portfolio investments	4.40%	4.60%